Method for the manufacture of a highly water resistant paper

ABSTRACT

PAPER WHICH HAS BEEN PROVIDED WITH A COATING COMPRISED OF A THERMOPLASTIC BINDING AGENT AND A PIGMENT IS TREATED WITH A MIXED AQUEOUS SOLUTION OF (1) BORIC ACID OR AN AMMONIUM OR METAL SALT THEREOF, (2) A MONOVALENT OR DIVALENT WATER SOLUBLE METALLIC SALT OR AN AMMONIUM SALT CHARACTERIZED BY A HIGH SALTING OUT PROPERTY, AND, OPTIONALLY PREFERABLY, (3) A WATER SOLUBLE METALLIC SALT WHICH IS A HIGHLY EFFECTIVE CHELATING AGENT, TO ENHANCE THE WATER RESISTANCE OF SUCH COATED PAPER.

United States Patent Office 3,740,253 Patented June 19, 1973 3,740,253 METHOD FOR THE MANUFACTURE OF A HIGHLY WATER RESISTANT PAPER Tatsuaki Hattori, 1621 Sakazu, Knrashiki, Japan No Drawing. Filed Dec. 9, 1969, Ser. No. 883,243 Claims priority, application Japan, Dec. 13, 1968, 43/ 91,815 Int. Cl. B44d 1/44; B32b 7/10 US. Cl. 117-62 Claims ABSTRACT OF THE DISCLOSURE Paper which has been provided with a coating comprised of a thermoplastic binding agent and a pigment is treated with a mixed aqueous solution of (1) boric acid or an ammonium or metal salt thereof, (2) a monovalent or divalent water soluble metallic salt or an ammonium salt characterized by a high salting out property, and, optionally preferably, (3) a water soluble metallic salt which is a highly effective chelating agent, to enhance the water resistance of such coated paper.

BACKGROUND OF THE ENVENTION (1) Field of the invention This invention relates to a method for the manufacture of a coated paper, and, more particularly, it relates to an improved method for the manufacture of a highly water resistant coated paper characterized by the fact that the coating does not adhere to the calenders during the conventional processing thereof.

(2) Description of the prior art In the known methods for the manufacture of coated paper boards by coating the paper board with a coating composition containing a mineral pigment, and prior to lustering by a calender, the coated surface is normally wet with water in order to fiuidize the coating layer such that a gloss is imparted to the coating surface without much reduction in the thickness of the paper board.

Before the water treatment, however, the coated paper is subjected to a drying operation with hot air at a temperature of about 100 C. so that the continuous Water treatment takes place at a temperature within the range of about 60 C. to 70 C. Consequently, when the coating composition is comprised of either a thermoplastic binding agent or a water soluble thermoplastic binding agent, it is frequently observed that the coating layer will adhere to the calender during the aforesaid water treatment.

Recently, polyvinyl alcohol (hereinafter referred to as PVA) has been used at least in part as a binding agent for the mineral pigment in place of casein which was theretofore conventionally employed. The greatest disadvantage of using PVA as the pigment binding agent in the manufacture of coated paper board resides in the tendency of same to adhere to a calender roll. Because PVA is a highly hydrophilic macromolecule it is readily penetrated with water during the calendering treatment, while the coating layer impregnated with a PVA binding agent exhibits a relatively great degree of tackiness at elevated temperatures.

In order to obviate the above disadvantages, it has been proposed to add either boric acid or borax to the water treatment of the coated broad comprised of a PVA binding agent such as to render the PVA rapidly insoluble. Paper provided with a coating comprised of PVA and treated with either boric acid or borax exhibits relatively good water resistance at room temperature. On the other hand, it has been appreciated that the water resistance of a PVA coated paper during a water treatment at a temperature in the range of about C. to C. is considerably insufficient.

SUMMARY OF THE INVENTION Therefore, a primary object of the present invention is to obviate the above and other disadvantages and difiiculties and to provide a new method for the manufacture of a highly water resistant coated paper characterized by the absence of adhesion of the coating layer to the calender at elevated temperatures. Another principal object of the invention is to provide an improved method for the manufacture of a highly water resistant coated paper and wherein the paper which is coated with a coating composition comprised of a pigment and PVA is treated with a mixed aqueous solution of boric acid or an ammonium or metal salt thereof, and a monovalent or divalent water soluble metallic salt or ammonium salt characterized by a high salting out property.

Another object of the invention is to provide a new method for the manufacture of highly water resistant coated paper in which the paper is coated with a composition comprised of pigment and PVA and is treated with a mixed aqueous solution of boric acid or an ammonium or metal salt thereof, a monovalent or divalent water soluble metallic salt or an ammonium salt characterized by a high salting out property, and a water soluble metallic salt which is a highly effective chelating agent.

Other objects, features and advantages of the invention will become apparent from the description which follows:

Briefly, the improved method for the manufacture of a highly water resistant coated paper according to the invention has for its salient characteristic treating the paper which is coated with a composition comprised of pigment and PVA with a mixed aqueous solution of boric acid or a metal salt thereof, (hereinafter referred to as the primary component), and a monovalent or divalent water soluble metallic salt or an ammonium salt characterized by a high, preferably a very high salting out property (hereinafter referred to as the secondary component), or the foregoing primary and secondary components in combination with a water soluble metallic salt which is a highly effective chelating agent (hereinafter referred to as the third component).

DESCRIPTION OF THE PREFERRED EMBODIMENT When a very highly water resistant coated paper is required, it is preferred to treat the coated paper, after finishing, with the three component mixed aqueous solution, namely, the mixed aqueous solution of the primary and secondary components which is additionally comprised of a water soluble metallic salt which is a highly effective chelating agent. Moreover, the content by weight of the aforesaid three components in the mixed aqueous solution will be about 1 to 10%, preferably about 1 to 5%, of the primary component, about 3 to 20%, preferably about 3 to 10%, of the secondary component, and about 1 to 10% preferably about 1 to 5%, of the third component.

In the present invention, the boric acid or ammonium or metal salt thereof of the primary component, for example, boric acid, borax, ammonium borate, potassium borate and sodium metaborate are quite effective. Boric acid, borax and potassium'borate are especially effective. By the monovalent or divalent water soluble metallic or ammonium salt of the secondary component there are intended the water soluble salts of citric acid, tartaric acid, sulfuric acid, carbonic acid, phosphoric acid and the like with metals such as lithium, sodium, potassium, and magnesium, or in the form of their ammonium salts. Sodium sulphate, sodium citrate and magnesium sulphate are especially effective. Further, by water soluble metallic salt which is a highly effective chelating medium there are intended the divalent to tetravalent water soluble metal salts of copper, zinc, aluminum, tin, titanium, zirconium, lead, chromium, iron, nickel and cobalt. Zinc sulphate, aluminum sulphate, zirconyl ammonium carbonate, and stannic chloride are especially effective.

The PVA according to the invention comprises partially or wholly saponified PVA having a degree of polymerization of from about 100 to 3000 with a degree of saponification of about 45 to 100 mol. percent, said PVA bearing carboxyl groups, said PVA containing sulfuric acid radicals, said PVA containing phosphoric acid radicals, said PVA containing straight or branched chain saturated fatty acid radicals and the like.

Although either the two or three component mixed aqueous solutions according to the invention provide for an eflicient coating of the substrate, it is preferred to employ a surfactant in addition to the said mixed aqueous solutions in order to further enhance the efficiency of coating. Moreover, the effect of imparting high water resistance is maintained even when the paper is coated with a composition comprised of, in addition to the PVA as the pigment binding agent, added binding agent such as starch, carboxymethyl cellulose or a latex.

In order to further illustrate the present invention and the advantages thereof, the following syecific examples are given, it being understood that same are only intended to be illustrative and in no wise limitative.

Example I Considering the fact that the tackiness of a coating layer impregnated with PVA will cause adhesion to a calender,

' the dependency of water absorption and swelling of the calendering agent according to the invention to temperature has 'been examined. For this purpose, a film of a thickness of 0.05 mm. comprised of PVA 117 having a degree of polymerization of 1700 and a degree of saponi- 'fication of 98.5 mol percent was immersed in the calendering liquid for a period of minutes to determine a swelling rate per area of film. Data are presented in the following Table I.

As is shown in the following Table I, Na SO reduces the degree of swelling of the PVA within the extended ranges of temperatures, while the boric acid and the borax reduce the degree of swelling of the PVA at a low temperature. Moreover, it has been surprisingly found that the use of boric acid and borax with Na SO exhibits a synergistic reduction in the degree of swelling of the PVA at a low temperature. The drawback of a high degree of swelling of the boric acid and borax at elevated temperatures may be solved by decreasing the amount of the boric acid and the borax and with use of N32804- borax Na SO mixed alkali aqueous solution according to the invention are more effective in reducing the degree of swelling of the PVA. Moreover, it will also be appreciated that N21 SO alone, although reducing the degree of swelling, is not the most preferred calendering agent because of its poor water resistance.

Example II 100 parts of clay were dispersed into water to obtain a slurry to which were subsequently added 5 parts of a 20% aqueous solution of PVA having a degree of polymerization of 550 with a degree of saponificat-ion of 88 mol. percent, in pure form, and 10 parts of styrenebutadiene copolymer latex, in solid form, such additions being under agitation to obtain a coating composition having a concentration of 40%. 17 g./m. of the resulting coating composition in the solid form was applied to paper, followed by a drying operation for two minutes at a temperature of about C. The coated paper was then subjected to super calendering.

To the thus prepared coated paper, upon heating at a temperature of 70 0., there was applied a treating agent in an amount of 0.03 cc. per 2 cm. of the coating surface and immediately thereafter another coated paper was surmounted thereon such that the coated surfaces were face-to-face with each other. The superposed coated papers were loaded with a weight of 170 g. for a predetermined period of time and after removal of the weight the tensile strength of the paper was measured As is shown in the above Table II, it will be appreciated that the three component mixed aqueous solutions according to the invention provide for considerably reduced tackiness.

Example III To coated papers obtained as in Example II there were applied the following aqueous treating solutions in amounts of 20 g./m. to each and, after a super calendering operation was eflected, the physical properties of the coated papers were measured. It was ascertained that TABLE I Areal degree of swelling 1 (percent) at- Calendering agent 30 C. 45 C. 60 0. 70 C.

Comparison Water 113. 8 122 Do Aqueous solution of 5% borax. 113 116. 5 127. 3 156 Do. Aqueous solution of 5% NazSO4 113 113. 8 115. 5 119. 3 Do... Aqueous solution of 10% N a 80, 109. 5 110. 8 111. 0 113. 3 Do..- Aqueous solution of 5% boric acid..-" 107. 5 108. 5 112.0 129. 3 Do Aqueous solution of 5% ZnSO 107. 5 108. 5 112. 0 129. 3

Invention Aqueous solution of 5% boric acid.-." 107. 0 108. 0 109. 0 118. 0 Do... Aqueous solution of 5% Net-2304 107. 0 108.0 109. 0 118. 0 Do Aqueous solution of 5% borax. 111. 0 111. 5 116. 0 126. 5 Do... Aqueous solution of 5% N82S04 111. 0 111.5 116.0 126. 5 Do... Aqueous solution of 2.5% boric acid- 107. 5 108. 8 109. 8 114. 0 Do... Aqueous solution of 2.5% ZnSOl 107. 5 108. 8 109. 8 114. 0 Do Aqueous solution of 5% NazSO4 107.5 108. 8 109. 8 114. 0

. Area of film after swelling Areal degree of swelling Area of film before XlOO.

From the above data, it will be appreciated that the boric acid NA SO mixed aqueous solution, the boric water resistance, as Well as the other general physical properties of all aqueous treating solutions according to acid Na SO -ZnSO mixed aqueous solution, and the 75 the invention were excellent.

Specifics as to the aqueous treating solutions examined:

(1) Aqueous solution of 5% Na SO -2.5% ZnSO -2.5%

boric acid (2) Aqueous solution of 5% Na SO.;-2.5% Al (SO' boric acid (3) Aqueous solution of 5% Na SO -5% borax (4) Aqueous solution of 5% 'Na SO -2.5% SnCl -2.5%

boric acid (3) Aqueous solution of 5% Na SO -2.5% CuCl -2.5%

boric acid Example IV In the same manner as in Example II, the tackiness of the coated papers, after 15 seconds of application of the aqueous treating solutions, was measured with respect to the following aqueous treating solutions.

Specifics as to the aqueous treating solutions examined:

(1) Aqueous solution of 5% Na SO -3% boric acid- 3%ZnSO (2) Aqueous solution of 5% sodium citrate-3% boric acid-3% Al (SO (3) Aqueous solution of 5% potassium tartrate-3% boric acid (4) Aqueous solution of Na PO -3% boric acid- 3% titanium lactate (5) Aqueous solution of 5% Na CO -3% boric acid-3% NaAlO (6) Aqueous solution of 5% sodium oxalate-3% potassium borate (7) Aqueous solution of 5% (NH SO -3% boric acid- All measurements indicated a reduced tackiness less than the precision of the Schopper tensile machine, which corroborates that the aqueous treating solutions according to the present invention are quite superior.

Since the invention is capable of considerable modification and variation from the details given above, any change which conforms to the spirit of the invention is intended to be included in the scope of the appended claims.

What is claimed is:

1. In the method for the manufacture of a highly water resistant paper provided with a coating comprised of a polyvinyl alcohol as a binding agent and a pigment, the improvement comprising treating such coated paper with a mixed aqueous solution of (1) about 1 to 10 weight percent of boric acid or an ammonium or metal salt thereof, (2) about 3 to 20 weight percent of a monovalent water soluble metallic salt characterized by a high salting out property and being selected from the group consisting of salts of a cation selected from the group consisting of lithium, sodium, potassium and ammonium and an anion selected from the group consisting of citric, tartaric, sulcfuric, carbonic and phosphoric acids, and (3) about '1 to 10 weight percent of a divalent to tetravalent water soluble metal salt of a metal selected from the group consisting of copper, zinc, aluminum, tin, titanium, zirconium, lead, chromium, iron, nickel and cobalt.

2. The method as defined by claim 1, wherein the component (l) is boric acid.

3. The method as defined in claim 1, wherein the additives in the mixed aqueous solution consist essentially of the additives ('1), (2) and (3).

4. The method as defined by claim 1, wherein the mixed aqueous solution is comprised of boric acid, sodium sulphate and zinc sulphate.

5. The method as defined by claim 1, wherein the coating is further comprised of a member selected from the group consisting of starch, carboxymethyl cellulose and a latex.

6. The method as defined by claim 1, wherein the content by weight of the component (11) in the mixed aqueous solution ranges from between about 1 and 5%, and the content by weight of the component (2) ranges from between about 3 and 10%.

7. The method as defined by claim 6, wherein the content by weight of the component (3) in the mixed aqueous solution ranges from between about 1 and 5%.

8. The method as defined by claim 1, wherein the mixed aqueous solution is further comprised of a surfactant.

9. The method as defined by claim 1, wherein the component 3) is a member selected from the group consisting of zinc sulphate, aluminum sulphate, zirconyl ammonium carbonate and stannic chloride.

10. A highly water resistant paper substrate provided with a coating comprised of a thermoplastic binding agent and a pigment which has been treated according to the process as defined by claim 1.

References Cited UNITED STATES PATENTS 3,476,582 11/1969 Imoto et a1 117-155 UA 3,481,764 12/1969 Matsumoto et al. 1'l7155 UA 3,438,808 4/ 1969 Hawkins et al. 117155 UA 3,429,735 2/ 1969 Bridgeford 117155 UA 3,218,191 11/1965 Domanski 117-155 UA 3,303,155 2/1967 Peterson et al. 117-155 UA 3,650,805 3/1972 Imoto et al. 117-455 3,578,485 5/1971 Imoto et al 1'17--155 UA OTHER REFERENCES Chemical Abstracts, vol. 68, 51121 (1968).

WILLIAM D. MARTIN, Primary Examiner M. SOFOCLEOUS, Assistant Examiner US. Cl. X.R. 117-15 5 UA 

